Most sheets of electrical steel are assembled into a product by welding or rivetting. However, welding fixation may result in a short circuit of edge portion of iron core, thus insulating property decreases. Further, various problems occur due to thermo-deformation, such as degradation of magnetic characteristics. In addition, when welding, burning of coating film generates harmful gases, and cohesion force is also poor. The welding and rivetting methods are not suitable to many mini electric machines thereby. In this case, it is an attractive fixation method to bond sheets of electrical steel to form an iron core. If an adhesive is applied onto each sheet of electrical steel cut off, production efficiency is very low, and the bonding is difficult to control and hardly homogeneous. Thus, to apply an adhesive directly onto surface of steel sheet in roll to form a self-bonding coat will increase subsequent process efficiency greatly.
The electrical steel plate thus obtained can be stored in sheet or roll form for a long time before being used to make an iron core. During storage, the steel plate does not rust, and no problem occurs such as the coating film breaks up. Thereafter, upon heating and pressing the coated surface at any time as desired, an iron core with excellent properties such as high bond strength between layers may be obtained.
Thyssen Krupp Steel EBG Company (Germany) has developed a steel plate with a self-bonding coat, which has multiple properties simultaneously, such as insulating property, corrosion resistance, improved punching property, and property of bonding with each other. When such steel sheets are assembled into an iron core product, they need no welding or rivetting any more, and avoid generation of harmful gases on welding. Further cohesion force between sheets is high enough. The product thus obtained may be used as a long stator iron core of Maglev.
However, the self-bonding coating used by EBG Company is a solvent-based product. A large amount of solvent vaporizes during the baking of the coating. In order to eliminate pollution to the surrounding, an incinerator is needed to burn out the solvent. Furthermore, any naked-flame baking device is not suitable for this product.
An aqueous heat-adhesive coat composition is proposed by Kawasaki (JP 6-21274). It comprises an emulsion of thermoplastic synthesis resin, an emulsion of epoxy resin, a curing agent and a film-forming agent. The composition may be applied onto the surface of steel sheet, and form a self-bonding coat.
Since the composition is formed by mixing an emulsion of thermoplastic resin and an emulsion of epoxy resin, which are not compatible completely with each other, the coat resulted thereby is not homogeneous, not transparent, and has a poor peel strength and shear strength. Further, in order to form a continuous coat without any crack, it is necessary to add a large amount of film-forming agent. This results in significant reduction of peel strength and adhesion strength of the coat. Furthermore, the liquid coating must be formulated in situ before application. It has a short shelf-life and is inconvenient for application.